If you want almost guaranteed success with cuttings just bank up a little soil about an inch higher than ground level around the penstemmon. In a few weeks the stems will layer themselves into this soil and root into it. You can then gently pull off rooted stems and pot them up or plant them out in their final position immediately.

Penstemmon's are not entirely hardy in very hard winters in the UK and young plants are at the most risk as they havent developed woody stems so I always plant them about 2 inches lower in the ground than they were in the pot I bought them in.

This gives 2 inches extra frost protection and often ,even if top growth has all been killed by frosts they'll put out new shoots from lower down later in spring.